The developers behind the popular open-source Web server software Nginx have released updates to patch a serious vulnerability.

Nginx Tuesday announced the release of nginx-1.4.1 -- as well as "development version" nginx-1.5.0 -- to fix a buffer-overflow vulnerability that attackers could exploit to execute arbitrary code on a Ngnix server and completely compromise it. In a security advisory issued Tuesday, Nginx said the bug is present in Nginx versions 1.3.9 and 1.4.0. "The problem is fixed in nginx 1.5.0 [and] 1.4.1," it said.

The vulnerability rates as "highly critical," according to a security advisory issued by vulnerability research firm Secunia. "The vulnerability is caused due to an error within [a] function ... when parsing an HTTP chunk and can be exploited to cause a stack-based buffer overflow," it said.

Nginx -- pronounced "engine X" -- is an open-source Web server, reverse proxy server, and load balancer designed for a large number of concurrent connections and high levels of performance but with a low memory footprint. It runs on Unix, Linux, Solaris and Windows, as well as AIX, BSD variants, HP-UX and Mac OS X.

Nginx is now the third most popular HTTP Web server software, behind Apache and Microsoft ISS, although its popularity continues to increase. "Nginx reached a new milestone this month: it is now used by more than 100M websites, and within the million busiest websites has overtaken Microsoft IIS to take second place with a market share of 13.5%," said a May 2013 Web server report released by Netcraft.

"Overall, Nginx's market share now stands at 15.5%, just 1.2 percentage points behind Microsoft, helped by a growth of 8.3M sites this month," it said.

The growing popularity of Nginx, however, has made it a target for attackers. Notably, the developers behind the Cdorked malware that targets Linux systems running Apache HTTP server software recently updated the malware to exploit Nginx, as well as open-source Lighttpd ("lighty") Web server software.

To date, Cdorked infections have been confirmed in about 400 Web servers, 50 of which rank in the Alexa index of the top 100,000 websites. But security researchers don't yet know how attackers are infecting servers with the backdoor malware.

"We still don't know for sure how this malicious software was deployed on the Web servers," said Marc-Etienne M. Leveille, a malware researcher at security firm ESET, in a blog post. "We believe the infection vector is not unique. It cannot be attributed solely to installations of cPanel because only a fraction of the infected servers are using this management software."

"One thing is clear, this malware does not propagate by itself and it does not exploit a vulnerability in a specific software," he said. "Linux/Cdorked.A is a backdoor, used by [a] malicious actor to serve malicious content from legitimate websites."

Interestingly, the malware "is even more stealthy than we first thought," he said. "By analyzing how the attackers are configuring the backdoor, we found it will not deliver malicious content if the victim's IP address is in a very long list of blacklisted IP ranges, nor if the victim's Internet browser's language is set to Japanese, Finnish, Russian and Ukrainian, Kazakh or Belarusian." In those cases, the malware is instead set to redirect users to a "page with links to pornographic websites," said Leveille.

ESET researchers have also clarified the relationship between CDorked and the Apache-targeting Darkleech (aka Chapro) malware attacks, which has continued to intensify in recent months. "While both target Apache servers, they are distinct pieces of code and send visitors to different instances of the Blackhole kit," said ESET malware researcher Sébastien Duquette. "However this does not change the fact that this trend is quite concerning."

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